There has been proposed a power supply that includes a DC-DC converter configured to be controlled so as to output a constant current. This power supply is used, for example, for lighting light-emitting diodes in a headlight device (see for example, JP2011-113642A). In a case where the power supply is used in a headlight device of a vehicle, an in-vehicle battery is used as a power source of the DC-DC converter.
It is notable that, in a case where the DC-DC converter is supplied energy from the in-vehicle battery and the in-vehicle battery is also connected to another load (such as a starter), the supplied voltage (input voltage to the DC-DC converter) may temporarily decrease when this another load is operated (for example, when an engine of the vehicle is activated).
The DC-DC converter is therefore operated so as to increase an output current thereof during a period in which the supplied voltage is decreased, so that the reduction of the output current of the DC-DC converter caused by the decrease of the supplied voltage can be compensated. In detail, for example, the ON duty ratio of a switching device of the DC-DC converter, which may be a boost converter (boost chopper) or a flyback converter, is increased during the period in which the supplied voltage is decreased.
However, when the input voltage to the DC-DC converter temporarily decreases and then returns to an original voltage, the output current of the DC-DC converter to a load possibly increases excessively due to the delay of the control in the DC-DC converter.